Re: [PATCH v3 4/5] common/domain: add a domain context record for shared_info...

[Jan Beulich <jbeulich@suse.com>]

On 19.05.2020 17:21, Paul Durrant wrote:
>> From: Jan Beulich <jbeulich@suse.com>
>> Sent: 19 May 2020 15:08
>>
>> On 14.05.2020 12:44, Paul Durrant wrote:
>>> --- a/xen/include/public/save.h
>>> +++ b/xen/include/public/save.h
>>> @@ -73,7 +73,16 @@ struct domain_save_header {
>>>  };
>>>  DECLARE_DOMAIN_SAVE_TYPE(HEADER, 1, struct domain_save_header);
>>>
>>> -#define DOMAIN_SAVE_CODE_MAX 1
>>> +struct domain_shared_info_context {
>>> +    uint8_t has_32bit_shinfo;
>>> +    uint8_t pad[3];
>>
>> 32-(or 16-)bit flags, with just a single bit used for the purpose?
>>
> 
> I debated that. Given this is xen/tools-only would a bit-field be acceptable?

Looking at domctl.h and sysctl.h, the only instance I can find is a
live-patching struct, and I'd suppose the addition of bitfields there
was missed in the hasty review back then. While it might be
acceptable, I'd recommend against it. It'll bite us the latest with
a port to an arch where endianness is not fixed, and hence may vary
between hypercall caller and callee. The standard way of using
#define-s looks more future proof.

Jan